1. Field of the Invention
The present invention relates to apparatus and methods for removal of heat from electronic devices. In particular, the present invention relates to a thermal interface comprising a grid frame coated with a thermal interface material, wherein the coated grid frame is disposed between a heat generating source, such as a microelectronic die, and a heat dissipation device, such as a heat spreader or heat sink.
2. State of the Art
Higher performance, lower cost, increased miniaturization of integrated circuit components, and greater packaging density of integrated circuits are ongoing goals of the microelectronic and computer industry. As these goals are achieved, microelectronic dice become smaller. Accordingly, the density of power consumption of the integrated circuit components in the microelectronic die has increased, which, in turn, increases the average junction temperature of the microelectronic die. If the temperature of the microelectronic die becomes too high, the integrated circuits of the microelectronic die may be damaged or destroyed.
Various apparatus and techniques have been used and are presently being used for removing heat from microelectronic dice. One such heat dissipation technique involves the attachment of a heat dissipation device to a microelectronic die. FIG. 4 illustrates an assembly 200 comprising a microelectronic die 202 (illustrated as a flip chip) physically and electrically attached to a carrier substrate 204 by a plurality of solder balls 206. A back surface 216 of a heat dissipation device 208 may be attached to a back surface 212 of the microelectronic die 202 by a thermally conductive adhesive or solder 214. The heat dissipation device 208 may be a heat pipe, as known in the art, or a heat slug constructed from a thermally conductive material, such as copper, copper alloys, aluminum, aluminum alloys, and the like.
However, the use of a rigid thermally conductive adhesive or solder 214 can cause stresses in the microelectronic die 202 due to a mismatch between coefficients of thermal expansion (xe2x80x9cCTExe2x80x9d) of the heat dissipation device 208 and the microelectronic die 202 as the microelectronic die 202 heats to a normal operating temperature when on and room temperature when off. Stresses due to CTE mismatch increase the probability that cracks will initiate and propagate in the microelectronic die 202, which may cause the failure of the microelectronic die 202. Furthermore, in order to get the solder materials to adhere to the microelectronic die back surface 212 and the heat dissipation device back surface 216, a gold coating may to be applied to both surfaces, which is prohibitively expensive.
In another known embodiment as shown in FIG. 5, a pin grid array-type (xe2x80x9cPGAxe2x80x9d) microelectronic die 222 is placed in a socket 224 mounted on the carrier substrate 204, wherein pins 226 extending from the PGA microelectronic die 222 make electrical contact with conductive vias 228 in the socket 224. The socket 224 is, in turn, in electrical contact (not shown) with the carrier substrate 204. The heat dissipation device 208 (shown as a finned heat sink having a plurality of fins 232) is kept in contact with the microelectronic die 222 with a spring clip 234 which spans the heat dissipation device 208 and connects to the socket 224. A conductive grease 236 is placed between the microelectronic die 202 and the heat dissipation device 208. This configuration virtually eliminates problems with CTE mismatch.
It is also known that the conductive grease 236 of FIG. 5 may be replaced with a phase-change material or matrix. Such materials are in a substantially solid phase (paste-like consistency) when cool (i.e., room temperature). When heated (brought to operating temperatures), the phase-change material changes to a substantially liquid phase (grease-like consistency) which allows the phase-change material to conform to surface irregularites of mating surfaces (when in a solid phase the phase change material is not able to conform to all microwarpages). Therefore the liquid phase has better contact properties which results in a higher heat dissipation compared to the solid phase. One such material is Thermflow(trademark) T443, consisting of a phase-change material within a fiberglass carrier (mat/pad) available from Chomerics, Woburn, Mass., USA.
However, as the size or xe2x80x9cfootprintxe2x80x9d of microelectronic dice decreases, the contact area between the microelectronic die and the heat dissipation device decreases, which reduces the area available for conductive heat transfer. Thus, with a decrease of the size in the microelectronic die, heat dissipation from the heat dissipation device becomes less efficient. Furthermore, as the microelectronic die power is increased, the heat source upper temperature specifications decreases, or the external ambient temperature specification increases. Thus, every area of thermal performance must be examined for any possible enhancement. One such area is the interface between the microelectronic die and the heat dissipation device. As microelectronic dice become smaller, the heat transfer properties of the interface becomes a greater factor. Thus, currently available thermally conductive adhesives, greases, and phase-change materials generally are quickly becoming bottlenecks to heat dissipation.
Therefore, it would be advantageous to develop techniques and apparatus to improve the efficiency of heat transfer at an interface between a heat source and a heat dissipation device.